TRUNCATE

Name

Synopsis

Description

TRUNCATE quickly removes all rows
from a set of tables. It has the same effect as an unqualified
DELETE on each table, but since it does
not actually scan the tables it is faster. Furthermore, it
reclaims disk space immediately, rather than requiring a
subsequent VACUUM operation. This is
most useful on large tables.

Parameters

name

The name (optionally schema-qualified) of a table to
truncate. If ONLY is specified
before the table name, only that table is truncated. If
ONLY is not specified, the table
and all its descendant tables (if any) are truncated.
Optionally, * can be specified
after the table name to explicitly indicate that descendant
tables are included.

RESTART IDENTITY

Automatically restart sequences owned by columns of the
truncated table(s).

CONTINUE IDENTITY

Do not change the values of sequences. This is the
default.

CASCADE

Automatically truncate all tables that have foreign-key
references to any of the named tables, or to any tables
added to the group due to CASCADE.

RESTRICT

Refuse to truncate if any of the tables have foreign-key
references from tables that are not listed in the command.
This is the default.

Notes

You must have the TRUNCATE privilege
on a table to truncate it.

TRUNCATE acquires an ACCESS EXCLUSIVE lock on each table it operates
on, which blocks all other concurrent operations on the table.
When RESTART IDENTITY is specified, any
sequences that are to be restarted are likewise locked
exclusively. If concurrent access to a table is required, then
the DELETE command should be used
instead.

TRUNCATE cannot be used on a table
that has foreign-key references from other tables, unless all
such tables are also truncated in the same command. Checking
validity in such cases would require table scans, and the whole
point is not to do one. The CASCADE
option can be used to automatically include all dependent tables
— but be very careful when using this option, or else you might
lose data you did not intend to!

TRUNCATE will not fire any ON DELETE triggers that might exist for the
tables. But it will fire ON TRUNCATE
triggers. If ON TRUNCATE triggers are
defined for any of the tables, then all BEFORE TRUNCATE triggers are fired before any
truncation happens, and all AFTER
TRUNCATE triggers are fired after the last truncation is
performed and any sequences are reset. The triggers will fire in
the order that the tables are to be processed (first those listed
in the command, and then any that were added due to
cascading).

Warning

TRUNCATE is not MVCC-safe
(see Chapter 13 for general
information about MVCC). After truncation, the table will
appear empty to all concurrent transactions, even if they
are using a snapshot taken before the truncation
occurred. This will only be an issue for a transaction
that did not access the truncated table before the
truncation happened — any transaction that has done so
would hold at least an ACCESS
SHARE lock, which would block TRUNCATE until that transaction completes.
So truncation will not cause any apparent inconsistency
in the table contents for successive queries on the same
table, but it could cause visible inconsistency between
the contents of the truncated table and other tables in
the database.

TRUNCATE is transaction-safe with
respect to the data in the tables: the truncation will be safely
rolled back if the surrounding transaction does not commit.

When RESTART IDENTITY is specified,
the implied ALTER SEQUENCE RESTART
operations are also done transactionally; that is, they will be
rolled back if the surrounding transaction does not commit. This
is unlike the normal behavior of ALTER
SEQUENCE RESTART. Be aware that if any additional sequence
operations are done on the restarted sequences before the
transaction rolls back, the effects of these operations on the
sequences will be rolled back, but not their effects on
currval(); that is, after the
transaction currval() will continue
to reflect the last sequence value obtained inside the failed
transaction, even though the sequence itself may no longer be
consistent with that. This is similar to the usual behavior of
currval() after a failed
transaction.

Examples

Truncate the tables bigtable and
fattable:

TRUNCATE bigtable, fattable;

The same, and also reset any associated sequence
generators:

TRUNCATE bigtable, fattable RESTART IDENTITY;

Truncate the table othertable, and
cascade to any tables that reference othertable via foreign-key constraints:

TRUNCATE othertable CASCADE;

Compatibility

The SQL:2008 standard includes a TRUNCATE command with the syntax TRUNCATE TABLE tablename. The clauses CONTINUE IDENTITY/RESTART
IDENTITY also appear in that standard, but have slightly
different though related meanings. Some of the concurrency
behavior of this command is left implementation-defined by the
standard, so the above notes should be considered and compared
with other implementations if necessary.